1. Field of the Invention
The present invention relates to an inkjet printing method and an inkjet printing apparatus which print images on a print medium by using an inkjet print head formed with nozzle arrays, each having ink ejecting nozzles arranged at high density.
2. Description of the Related Art
As information processing devices, such as computers and word processors, and communication devices have come into wide use, there are growing demands for output devices that output digital image information processed by the information processing devices onto a medium. As one such output device, an inkjet printing apparatus that forms an image by ejecting ink droplets to form dots on a print medium is rapidly finding its widespread use. To improve the printing speed and the resolution of printed images, this inkjet printing apparatus uses a print head that has a large number of ejection portions (also referred to nozzles) arranged in arrays, with each nozzle comprised of an ink droplet ejection opening, an ink path and a printing element or heater. There is a growing need in recent years for a capability to produce color printed images. Particularly, in producing photographic images, calls are increasing for reducing the volume of ink droplets to enhance the quality of printed images.
With a technological advance in recent years in enhancing the integration of nozzle arrays, the fabrication of a so-called elongate print head with highly dense nozzles is becoming a reality. This elongate print head can print on a print medium over an area of a greater width by its single scan than that possible with the conventional print head. This is considered a very promising technique in realizing a fast printing that has never been achieved before while maintaining as high a print quality as the conventional one. Active research is under way for further technical development. In general inkjet printing apparatus, it has been known that air currents are produced between the print head and a print medium during the printing operation.
In an inkjet printing apparatus using such an elongate print head, air currents are formed between the print head and the print medium during a printing operation, flowing around the highly dense wall of ejected ink. These wrapping air currents may in turn deflect the direction of ejected ink droplets, resulting in dot landing positions being deviated. As one method of preventing such image quality degradations, a technique is disclosed in Japanese Patent Laid-Open No. 2006-192892.
FIG. 8 shows a mask pattern used in a conventional printing apparatus. The mask pattern 100 is applicable to a so-called 2-pass printing method which completes an image in each print area by two scans complementing each other. In the conventional printing method using this mask pattern 100, the print area is divided into two areas at a predetermined interval in a nozzle-arrayed direction (direction of arrow α in FIG. 8). In one area, high printing ratio regions Hn are printed in a first scan of the 2-pass printing and then low printing ratio regions Ln whose print data is highly thinned are printed in a second scan. In the other area, the low printing ratio regions Ln are printed in the first scan and the high printing ratio regions Hn are printed in the second scan. With the two scans combined, a total of 100% printing ratio is achieved.
The use of the mask pattern 100, in which the thinning ratio is alternated between two levels, for example, from low to high, to low, to high and so on, forms gaps in a highly dense wall of ejected ink. More specifically, portions in the highly dense ink wall corresponding to the high thinning ratio regions in the mask pattern constitute gaps which are alternated with the dense ink wall portions corresponding to the low thinning ratio regions. These gaps allow air currents to pass through, reducing the amount of the wrapping air currents, which in turn minimizes deviations in dot landing position.
With the technique disclosed in Japanese Patent Laid-Open No. 2006-192892, large gaps need to be formed in the ink wall extending in the nozzle-arrayed direction for air currents to flow through the ink wall. In this method, the direction of scan of the print head as it prints the high printing ratio regions Hn and the low printing ratio regions Ln differs between the two areas.
Where the direction of scan, as the print head prints the high printing ratio regions and the low printing ratio regions, differ between the two areas, density unevenness may show in a printed image. For example, if the distance between a main droplet of ejected ink and a satellite, an extremely small ink droplet trailing the main droplet, differs between the forward scan direction and the backward scan direction, resulting in variations in dot-covered area ratio, there occurs a difference in the printed image density between the forward scan direction and the backward scan direction. This density difference is sometimes recognized as density unevenness.